It has long been recognized that if a vehicle entering a skid has the brakes cyclically applied there is a significantly improved chance of maintaining control of the vehicle. For many years the recommendation to the motoring public and professional drivers was that if their vehicle enters a skid, the brakes should be "pumped" to improve chances of maintaining control of the vehicle. This "pumping" action is intended to provide a cyclic application of fluid pressure to the individual wheel brake actuators. The more skilled professional driver, after becoming familiar with the peculiarities of his vehicle, was able to significantly reduce the chances of losing control of the vehicle during a skid condition. For the motoring public in general, and professional drivers not accustomed to a particular vehicle, the manually-controlled pumping procedure may produce an aggravated skid condition resulting in a complete loss of vehicle control.
Heretofore, prior art skid control valves have been provided which relieve a vehicle operator of the responsibility of properly applying cyclic pressure to a vehicle brake system in a skid condition. The prior art devices, upon sensing a skid condition, are cyclically operated to produce a pumping action application of fluid pressure to the individual wheel brake actuators. The cyclic pressure is applied to the wheel brake actuators for an axle so long as the skid condition exists. While prior art devices have generally been satisfactory, there still exists a need for improvement in several respects. The majority of prior art devices include various moving elements thereof which interface with both the primary or master cylinder fluid as well as the secondary fluid. Although sealing devices have heretofore been employed in such moving components to separate primary and secondary fluids, those sealing devices have proven ineffective over a period of time to isolate the primary and secondary fluids. When both the primary and secondary fluids act on the same sealing device, deterioration of that sealing element has heretofore been unavoidable. Further, adverse effects on the skid control apparatus as a whole will result due to the leakage of primary fluid into the secondary fluid system and vice versa. Moreover, with valves having moving members therein in liquid contact with more than one type of fluid, it is impossible to replace a moving part without involving both fluid systems. Therefore, there is a need for a skid control valve in which each moving member is in fluid contact with only one of the system fluids.
In the operation of some prior art devices, once an operator engages the brake and a skid condition is sensed, the operator is locked out of the system until the skid condition is corrected. That operation is not desirable in all situations. For example, should an operator engage the brake while traveling over a patch of ice, the force he applies to the brake pedal will be relatively light. If, after the skid control device becomes operative, the vehicle passes over the patch of ice and encounters dry pavement on which a greater braking force is required, the operator would be unable to effect the necessary increase in brake pressure until the anti-skid mechanism terminates its operation and the fluid path from the master cylinder to the wheel cylinder is reopened. Thus, there is a need for an anti-skid valve apparatus which provides the pumping effect required to overcome the skid, but at the same time allows the vehicle operator to reenter the operation when a greater wheel cylinder pressure is required.
Further, if for any reason the secondary fluid source does not provide sufficient secondary fluid pressure, the anti-skid device will not function properly. In those situations it is desirable to maintain the operation of the primary fluid system without the encumbrance of a non-functional secondary fluid system. Therefore, there is a need for an improved anti-skid valve apparatus which includes a device responsive to the pressure of the secondary fluid source to isolate the secondary fluid system from the skid control valve whenever the secondary fluid source pressure is below a predetermined value.